1. Field of the Invention
The present invention relates to a jitter measuring apparatus and a jitter measuring method for measuring timing jitter of a signal-under-test and a testing apparatus for testing a device-under-test.
2. Description of the Related Art
Conventionally, a method for comparing phase of a signal-under-test with that of a signal generated by delaying the signal-under-test by one period is known as a method for measuring jitter of a signal-under-test (T. Yamaguchi, “A Real-Time Jitter Measurement Board for High Performance Computer and Communication System”, ITC2004). In case of delaying the signal-under-test by one period, phases of two adjacent rising edges of the signal-under-test are compared and thus period jitter of the signal-under-test is measured.
The above method outputs voltage corresponding to the amount of the period jitter of each cycle by inputting the signal-under-test and the delayed signal-under-test to a phase comparator. Further, by integrating the voltage output by the phase comparator using an integrator and the like, it is possible to measure the timing jitter of the signal-under-test in real-time.
However, the conventional method for measuring jitter requires delaying a signal-under-test accurately. For example, in case of measuring period jitter of the signal-under-test, it is required to delay the signal-under-test by one period accurately. However, in case there is an error in the delay time, the period jitter amount detected by the phase comparator becomes larger than a true period jitter amount due to the error in delay. Further, the error in measurement is accumulated by the integrator and the timing jitter output by the integrator is saturated before long.
Thus, the conventional method for measuring jitter requires a delay circuit in order to generate an accurate delay time. However, since it is difficult to make a delay circuit with highly accurate delay time, for the conventional method for measuring jitter it is difficult to measure the jitter of the signal-under-test precisely without being affected by the delay time error.
The conventional measuring method compares phase of a signal-under-test with that of a delayed signal. Thus, in case period-jitter amplitude of the signal-under-test is larger than one period of the signal-under-test, sometimes phases are compared between edges which do not correspond to each other and thus the jitter cannot be measured accurately.